Dopamine hypothesis of schizophrenia
The dopamine hypothesis of schizophrenia or the dopamine hypothesis of psychosis is a model, attributing symptoms of schizophrenia (like psychoses) to a disturbed and hyperactive dopaminergic signal transduction. The model draws evidence from the observation that a large number of antipsychotics have dopamine-receptor antagonistic effects. The theory, however, does not posit dopamine overabundance as a complete explanation for schizophrenia. Rather, the overactivation of D2 receptors, specifically, is one effect of the global chemical synaptic dysregulation observed in this disorder.
Some researchers have suggested that dopamine systems in the mesolimbic pathway may contribute to the 'positive symptoms' of schizophrenia (whereas problems with dopamine function in the mesocortical pathway may be responsible for the 'negative symptoms', such as avolition and alogia.) Abnormal expression, thus distribution of the D2 receptor between these areas and the rest of the brain may also be implicated in schizophrenia, specifically in the acute phase. A relative excess of these receptors within the limbic system means Broca's area, which can produce illogical language, has an abnormal connection to Wernicke's area, which comprehends language but does not create it. Note that variation in distribution is observed within individuals, so abnormalities of this characteristic likely play a significant role in all psychological illnesses. Individual alterations are produced by differences within glutamatergic pathways within the limbic system, which are also implicated in other psychotic syndromes. Among the alterations of both synaptic and global structure, the most significant abnormalities are observed in the uncinate fasciculus and the cingulate cortex. The combination of these creates a profound dissymmetry of prefrontal inhibitory signaling, shifted positively towards the dominant side. Eventually, the cingulate gyrus becomes atrophied towards the anterior, due to long-Term Depression (LTD) and Long-Term Potentiation (LTP) from the abnormally strong signals transversely across the brain. This, combined with a relative deficit in GABAergic input to Wernicke's area, shifts the balance of bilateral communication across the corpus callosum posteriorly. Through this mechanism, hemispherical communication becomes highly shifted towards the left/dominant posterior. As such, spontaneous language from Broca's can propagate through the limbic system to the tertiary auditory cortex. This retrograde signaling to the temporal lobes, results in the parietal lobes not recognizing it as internal, resulting in the auditory hallucinations typical of chronic schizophrenia.
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